Searching for control channels in a wireless network

ABSTRACT

A user equipment (UE) may receive an OFDM signal having control channel elements (CCEs). The CCEs may be arranged in levels where a first level aggregates less CCEs than a second level. A processor may search for a control channel from control channel candidates that is comprised of the CCEs. A limited number of CCEs may be searched on the first level.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/928,749 filed Jul. 14, 2020 which is a continuation of U.S. patentapplication Ser. No. 15/876,917, filed Jan. 22, 2018, which issued asU.S. Pat. No. 10,716,063 on Jul. 14, 2020, which is a continuation ofU.S. patent application Ser. No. 15/050,011, filed Feb. 22, 2016, whichissued as U.S. Pat. No. 9,877,277 on Jan. 23, 2018, which is acontinuation of U.S. patent application Ser. No. 12/451,350, filed Mar.2, 2010, which issued as U.S. Pat. No. 9,271,266 on Feb. 23, 2016, whichis the U.S. national stage of International Application No.PCT/EP2008/055557, filed May 6, 2008, which claims the benefits ofEuropean Application No. 07107652.5, filed May 7, 2007, which areincorporated by reference as if fully set forth.

FIELD OF THE INVENTION

The present invention relates to control channels in communicationnetwork systems, and in particular to control channel allocation anddecoding e.g. in 3GPP (3^(rd) Generation Partnership Project) LTE (LongTerm Evolution) network systems.

BACKGROUND OF THE INVENTION

LTE technology, for example, defines a packet radio system, where allchannel allocations are expected to happen in short periods ofsub-frames. This is contrary to the older 3G systems, where dedicatedsignalling channels are necessary to be set up even for packet traffic.It is also different from WLAN (Wireless Local Area Network) type ofallocations, where each IP (Internet Protocol) packet transmissioncontains a transport header.

According to LTE technology, all allocations are signalled in SharedControl Channels, which are present in first multi-carrier symbols of asub-frame preceding multi-carrier symbols of data channels. The controlchannels are separately coded. That is, a downlink (or uplink) channelis divided into two separate parts, one for control and one for data.The data part (PDSCH) carries downlink (or uplink) data forsimultaneously scheduled users, while the control part (PDCCH) carries(among others) allocation information for the scheduled users.

SUMMARY OF THE INVENTION

The present invention provides methods and devices for reducing controlchannel decoding complexity as defined in the appended claims. Theinvention may also be implemented as computer program product.

According to the invention, a tree search for aggregated controlchannels is reduced in a systematic way, which will obtain a significantreduction of the number of decoding attempts at a UE (User Equipment)side, while still maintaining most of the scheduling flexibility in aneNB (evolved Node B), i.e. system spectral efficiency versus UEcomplexity trade-off is obtained.

According to the invention it is assumed that there will not be a largeamount of users having the same propagation conditions being scheduledat the same time. The tree reduction is obtained by putting somelimitations on the tree structure through specifications.

The UE utilizes the tree structure to reduce the decoding complexity inorder to save power. According to an embodiment of the invention, powerconsumption in the decoding/detection of an L1/L2 control channel can bereduced.

For the purpose of the present invention to be described herein below,it should be noted that

a user equipment may for example be any device by means of which a usermay access a communication network; this implies mobile as well asnon-mobile devices and networks, independent of the technology platformon which they are based;

a user equipment can act as a client entity or as a server entity interms of the present invention, or may even have both functionalitiesintegrated therein;

method steps likely to be implemented as software code portions andbeing run using a processor at one of the server/client entities aresoftware code independent and can be specified using any known or futuredeveloped programming language;

method steps and/or devices likely to be implemented as hardwarecomponents at one of the server/client entities are hardware independentand can be implemented using any known or future developed hardwaretechnology or any hybrids of these, such as MOS, CMOS, BiCMOS, ECL, TTL,etc, using for example ASIC components or DSP components, as an example;

generally, any method step is suitable to be implemented as software orby hardware without changing the idea of the present invention;

devices can be implemented as individual devices, but this does notexclude that they are implemented in a distributed fashion throughoutthe system, as long as the functionality of the device is preserved.

The present invention is not limited to LTE network systems, but can beapplied to any other communication systems requiring dynamic and fastchannel allocation, including systems where there will be multiple coderates for the control channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a tree structure with three allocated nodesin different levels of the tree.

FIG. 2 shows the three allocated nodes of FIG. 1 mapped to sub-carrierresources in a distributed manner.

FIG. 3 shows a combination of control channel elements to createaggregated control channel candidates.

FIG. 4 shows an example illustrating reduction of possible aggregationoptions for control channel candidates according to an embodiment of theinvention.

FIG. 5 shows a schematic block diagram illustrating functions of a userequipment and a network device according to an embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A Physical Downlink Shared Control Channel (PDSCCH) which carries (amongothers) allocation information for simultaneously scheduled users of acommunication network system is arranged to a tree structure consistingof multiple control channel elements as shown in FIG. 1 . Duringdecoding, a UE (User Equipment) will combine or aggregate the controlchannel elements to create different code blocks or control channelcandidates. Each code block is called a control channel candidate as itpotentially carries information for one MAC (Medium Access Control) ID.The MAC ID is used by the UE or by a group of UEs to detect the channel.At each level of the tree, each node represents a single control channelof a code block. The number of the control channels at the lowest levelof the tree is determined by the system bandwidth and number of OFDMsymbols n available for the largest code blocks, as well as the size ofthe control channel elements. In the example shown in FIG. 1 , n=3. Anynode of the tree, which is not occupied by a control channel in thislevel, is available for the next level of the tree as two controlchannels, each of which are half of the size of the control channel atthe parent node.

The system bandwidth consisting of a given number of sub-carrierresources may be divided to an integer multiple of the largest controlchannels. A given node of the tree i.e. a set of sub-carriers canconsist of one control channel of the largest code block, of up to twocontrol channels of the second largest code blocks or up to four controlchannels of the smallest code blocks.

Each control channel extends entirely over the first n OFDM symbols,which are available for the control channels. The control channels maybe distributed to the sub-carriers over the system bandwidth to maximizethe frequency diversity. For example, there are 4 distributed sets ofsub-carrier resources allocated per each code-block. This is illustratedin FIG. 2 .

In FIG. 1 , three allocated nodes CB1, CB2, CB3 in different levels ofthe tree structure are shown. FIG. 2 shows the three allocated nodesCB1, CB2, CB3 mapped to the sub-carrier resources in a distributedmanner. It should be noted that these mappings are only examples, andthat the mapping in general should provide frequency diversity byscattering over the system bandwidth.

As each control channel has to be uniquely identified by a MAC ID, itcan be combined to CRC (Cyclic Redundancy Code) by partly masking CRCbits with the MAC-ID. As the MAC ID is used for addressing both UEspecific control channels and common control channels, it is reasonableto define the MAC ID in a compatible way. Thus, reception of any controlchannel is possible by filtering control channels with the respectiveMAC ID. Error detection is available from the MAC ID masked CRC. Thelength of the MAC ID is matched to the C-RNTI (Cell Radio NetworkTemporary Identifier) length.

A receiver, e.g. the UE, includes means to receive symbols of theDownlink Shared Control Channel part of the sub-frame prior to receptionand processing of the symbols in the downlink and uplink Shared DataChannels. The receiver demodulates and decodes the sub-carriers of theOFDM symbols in which the receiver may search for a set of largest codeblocks, e.g. CB1 of FIG. 1 . As the Code Block is of known size and thesystem bandwidth is known, the receiver knows an integer multiple ofsub-carrier positions to search for a CB1. The reception, whethercorrectly detected or not, may be recognized by Cyclic Redundancy Checkdetector filtered by a receiver specific c-RNTI identity. For everymatch of CRC, to which the c-RNTI of the UE does not match, the receiverknows that the next higher level of tree is masked and not available.For every non-matched CRC check, the UE will continue decoding codeblocks (CB2) in the next higher level of the tree searching for a matchin two child nodes of the parent node. Further on, for every non-matchedCRC check, the UE will continue decoding code blocks (CB3) in the nexthigher level of the tree searching for a match in two child nodes of theparent node. The search continues until the UE has detected andcorrectly decoded all control channels, intended for its reception.

In addition to search signalling entries with its own receiver specificc-RNTI, the UE may have to search for common signalling entries bycommon identifiers.

The search in the tree may happen in any other order than from thelowest level node towards the higher level nodes. Depending on theapplied coding scheme, the receiver may process the nodes from thehighest level of nodes to the lower level of nodes. Further on, thereceiver may process the nodes in other arbitrary (or systematic) orderbased on some measures e.g. SINR (Signal Interference and Noise Ratio)quality of the candidate code block(s).

In the following it is assumed that only a single size of a node (i.e.control channel) at the highest level of the tree structure (level 3 inFIG. 1 ) is defined for a given bandwidth in a cell. The highest levelnode is referred to as “control channel element”. Aggregation ofmultiple control channel elements can be used to get larger payloadsand/or lower coding rate.

However, the aggregation of the control channel elements may require alarge number of decoding attempts from all the UEs that are listeningfor a possible allocation. An example of a control channel aggregationis shown in FIG. 3 .

From FIG. 3 it can be seen that an aggregation of even a relative lownumber of control channel elements will result in a rather high numberof decoding attempts for the UEs listening for resource allocations, andeach UE will listen for downlink allocations as well as uplinkallocations. In the example in FIG. 3 , there are 6 control channelelements, while the aggregation using a tree structure as shown in FIG.1 results in 10 potential control channel candidates. This is suboptimalregarding UE complexity, as a UE will have to decode the full amount ofcontrol channel candidates even if some of them are not scheduled.

In the following an embodiment of the invention will described ingreater detail.

FIG. 4 shows a kind of flat tree structure arising from FIG. 3 . FIG. 4illustrates potential control channel candidates for differentaggregation possibilities (both the white and grey areas). As can beseen from FIG. 4 , there is a total of 24 control channel elements(CCEs), which by default triggers 45 decoding attempts per linkdirection (i.e. downlink/uplink) allocation. In other words, inaggregation level 1 the 24 control channel elements each may form acontrol channel. In aggregation level 2, two control channel elementsmay be aggregated to form a control channel, in aggregation level 4,four control channel elements may be aggregated to form a controlchannel, and in aggregation level 8, eight control channel elements maybe aggregated to form a control channel.

According to an embodiment of the invention, the control channelstructure shown by the white and grey areas in FIG. 4 is put underlimitations, such that only the white aggregated control channelcandidates are available for scheduling. With this limitation, thenumber of decoding attempts is reduced to 15 (the grey areas are notdecoded in search for a control channel candidate), which corresponds toa reduction by a factor of 3. In other words, there are four controlchannel candidates in aggregation level 1, four control channelcandidates in aggregation level 2, four control channel candidates inaggregation level 4, and three control channel candidates in aggregationlevel 8.

With the above limitation put on the tree structure, schedulingflexibility is not reduced that much, based on the following arguments:

If there is a lot of user equipments close to an eNB scheduling thecontrol channels, which user equipments require only aggregation level1, the aggregated level 2 elements with reduced power can be used tohave more users due to the possibility of doing power balancing; in theexample shown in FIG. 4 , 9 favourable conditioned users can bescheduled using this approach. In other words, four control channels inaggregation level 1, two control channels in aggregation level 2, twocontrol channels in aggregation level 4, and one control channel inaggregation level 8 can be scheduled.

If a plurality of scheduled users is present at a cell edge (aggregationlevel 8), additional users cannot be scheduled anyway due to limitednumber of available control channel elements.

As the difference between aggregation layers is a factor of 2 and whenusing power balancing, to some extent there is flexibility to tradeaggregation and power between each other.

It should be noted that although the above description is given for anallocation tree for single link direction, the invention is also validfor the case where two trees, for uplink and downlink, respectively, arepresent.

Further, it should be noted that the number of possible control channelsat each layer is not important.

According to an embodiment of the invention, using an allocation rule,usage of the smallest control channel on all control channel elements isprohibited, while at the same time the smaller control channels areallowed to be combined to aggregated control channels with bettercoverage.

With the above approach, the number of decoding attempts that is neededby each UE can be reduced. The limitation of the tree is possible due tothe frequency diversity applied for all control channel elements, suchthat each CCE experiences same or similar channel conditions.

FIG. 5 shows a schematic block diagram illustrating a user equipment 10and a network device 20, such as an eNB, according to an embodiment ofthe invention.

The user equipment 10 comprises a receiving/transmitting section 11 anda decoding section 12. The receiving/transmitting section 11 receivessymbols from the network device 20, which comprises a receivingtransmitting section 21 transmitting the symbols and an allocationsection 22.

The allocation section 22 allocates control channels represented bynodes of a tree structure, each of the control channels comprising atleast one control channel element carrying information for a respectiveidentifier used to detect a control channel of the control channels,wherein the allocation is performed by limiting allocation of highestlevel control channels of the control channels, the highest levelcontrol channels being represented by nodes of the tree structure at ahighest level of the tree structure. For example, in FIG. 1 the highestlevel is shown by level 3. Referring to FIG. 4 , the highest level isrepresented by aggregation level 1.

The allocation section 22 may increase allocation of lower level controlchannels of the control channels, the lower level control channels beingrepresented by nodes of the tree structure at lower levels of the treestructure. For example, in FIG. 1 the lower levels are shown by levels 2and 1. Referring to FIG. 4 , the lower levels are represented byaggregation levels 2, 4 and 8.

The receiving/transmitting section 21 may transmit the allocated controlchannels as symbols to user equipments including the user equipment 10,by distributing the allocated control channels to sub-carriers over asystem bandwidth.

The higher level control channels may be combined to the lower levelcontrol channels. In other words, smaller control channels are allowedto be combined to aggregated control channels with better coverage.

The allocation section 22 may increase allocation more the lower thelevel of the tree structure.

The searching section 12 of the user equipment 10 searches for a controlchannel by decoding control channels represented by nodes of a treestructure, by using an identifier such as an MAC ID, CRC or c-RNTI, eachof the control channels comprising at least one control channel elementcarrying information for a respective identifier used to detect acontrol channel of the control channels, wherein the searching section12 limits the searching for highest level control channels of thecontrol channels, the highest level control channels being representedby nodes of the tree structure at a highest level of the tree structure.

The searching section 12 may increase the searching for lower levelcontrol channels of the control channels, the lower level controlchannels being represented by nodes of the tree structure at lowerlevels of the tree structure.

The receiving/transmitting section 11 may receive the control channelsas symbols from the network device 20.

The searching section 11 may begin the searching with lowest levelcontrol channels represented by nodes of the tree structure at a lowestlevel of the tree structure. For example, in FIG. 1 the lowest level isshown by level 1. Referring to FIG. 4 , the lowest level is representedby aggregation level 8.

It is to be noted that the network device 20 and user equipment 10 shownin FIG. 5 may have further functionality for working e.g. as eNodeB andUE. Here the functions of the network device and user equipment relevantfor understanding the principles of the invention are described usingfunctional blocks as shown in FIG. 5 . The arrangement of the functionalblocks of the network device and user equipment is not construed tolimit the invention, and the functions may be performed by one block orfurther split into sub-blocks.

According to an embodiment of the invention, on a transmitting side,control channels represented by nodes of a tree structure are allocated,each of the control channels comprising at least one control channelelement carrying information for a respective identifier used to detecta control channel of the control channels. The allocation is performedby limiting allocation of highest level control channels of the controlchannels, the highest level control channels being represented by nodesof the tree structure at a highest level of the tree structure. On areceiving side, a control channel is searched for by decoding theallocated control channels, wherein the searching is limited for thehighest level control channels.

It is to be understood that the above description is illustrative of theinvention and is not to be construed as limiting the invention. Variousmodifications and applications may occur to those skilled in the artwithout departing from the scope of the invention as defined by theappended claims.

What is claimed is:
 1. A method of operating a user equipment (UE), themethod comprising: receiving control channel information; and searchingcontrol channel elements for a control channel in the control channelinformation by decoding control channels comprising one or more controlchannel elements using an identifier, the one or more control channelelements being aggregated according to three or more aggregation levels,the one or more control channel elements at a highest aggregation levelcomprising a single control channel element and the one or more controlchannel elements at lower aggregation levels comprising a plurality ofconsecutive control channel elements, wherein the searching is limitedfor control channel elements at the highest aggregation level andwherein the searching increases more the lower the level of the three ormore aggregation levels.
 2. The method of claim 1, wherein searching fora control channel at a lower aggregation level is performed prior tosearching for a control channel at the highest aggregation level.
 3. Themethod of claim 1, wherein searching for a control channel at thehighest aggregation level is performed prior to searching for a controlchannel at a lower aggregation level.
 4. The method of claim 1, whereinsome of the control channels are decoded using t a cell radio networktemporary identifier (C-RNTI) associated with the UE and other controlchannels are decoded using a common identifier.
 5. The method of claim1, further comprising: determining a Signal to Interference and NoiseRatio (SINR) of received signals; and based at least in part on theSINR, determining to search for a control channel at a lower aggregationlevel prior to searching for a control channel at the highestaggregation level.
 6. The method of claim 1, wherein the searching beinglimited for a highest aggregation level and being increased for loweraggregation levels enables reduced power consumption of the UE.
 7. Adevice comprising: a receiver operable to receive control channelinformation; and circuitry operable to search control channel elementsfor a control channel in the control channel information by decodingcontrol channels comprising one or more control channel elements usingan identifier, the one or more control channel elements being aggregatedaccording to three or more aggregation levels, the one or more controlchannel elements at a highest aggregation level comprising a singlecontrol channel element and the one or more control channel elements atlower aggregation levels comprising a plurality of consecutive controlchannel elements, wherein the search is limited for control channelelements at the highest aggregation level and the search for controlchannel elements increases more the lower the level of the three or moreaggregation levels.
 8. The method of claim 1, wherein the controlchannel information is received from a network device.
 9. The device ofclaim 7, wherein the searching being limited for a highest aggregationlevel and being increased for lower aggregation levels enables reducedpower consumption of the device.
 10. The device of claim 7, wherein thecircuitry is further operable to search for a control channel at a loweraggregation level prior to searching for a control channel at thehighest aggregation level.
 11. The device of claim 7, wherein thecircuitry is further operable to search for a control channel at thehighest aggregation level prior to searching for a control channel at alower aggregation level.
 12. The device of claim 7, wherein some of thecontrol channels are decoded using a cell radio network temporaryidentifier (C-RNTI) associated with the device and other controlchannels are decoded using a common identifier.
 13. The device of claim7, further comprising: circuitry operable to determine a Signal toInterference and Noise Ratio (SINR) of received signals; and circuitryoperable to determine, based at least in part on the SINR, to search fora control channel element at a lower aggregation level prior tosearching for a control channel element at the highest aggregationlevel.
 14. The device of claim 7, wherein the control channelinformation is received from a network device.
 15. The device of claim7, wherein the device is a user equipment (UE).
 16. A non-transitorycomputer readable medium comprising a set of instructions that, whenexecuted by a processor of a user equipment (UE), causes the UE to:receive control channel information; and to search control channelelements for a control channel in the control channel information bydecoding control channels comprising one or more control channelelements using an identifier, the one or more control channel elementsbeing aggregated according to three or more aggregation levels, the oneor more control channel elements at a highest aggregation levelcomprising a single control channel element and the one or more controlchannel elements at lower aggregation levels comprising a plurality ofconsecutive control channel elements, wherein the search is limited forcontrol channel elements at the highest aggregation level and the searchfor control channel elements increases more the lower the level of thethree or more aggregation levels.
 17. The non-transitory computerreadable medium of claim 16, wherein searching for a control channel ata lower aggregation level is performed prior to searching for a controlchannel at the highest aggregation level.
 18. The non-transitorycomputer readable medium of claim 16, wherein searching for a controlchannel at the highest aggregation level is performed prior to searchingfor a control channel at a lower aggregation level.
 19. Thenon-transitory computer readable medium of claim 16, wherein some of thecontrol channels are decoded using a cell radio network temporaryidentifier (C-RNTI) associated with the UE and other control channelsare decoded using a common identifier.
 20. The non-transitory computerreadable medium of claim 16, wherein the set of instructions, whenexecuted by a processor of a user equipment (UE), causes the UE to:determine a Signal to Interference and Noise Ratio (SINR) of receivedsignals; and determine, based at least in part on the SINR, to searchfor a control channel at a lower aggregation level prior to searchingfor a control channel at the highest aggregation level.